Which group of vertebrates has the highest number of endangered species?

Which species has become extinct due to overexploitation by humans?

Many species like $Steller's$ sea cow and passenger pigeon have been driven to the brink of extinction. Which of the following describes this situation?

$A$ tree species in Mauritius failed to reproduce because of the extinction of a fruit-eating bird. Which one of the following was that bird?

The Russian species that became extinct in recent years is . . . . . . .

Which one of the following is $NOT$ a natural reason for loss of biodiversity?